Damage to the joints of the human body, due to arthritis, other degenerative diseases or trauma, often results in the need to replace the joint with a prosthetic device. In the case of hip replacement surgery such devices include a cup which fits into the hip socket, and replaces the acetabulum, and a ball attached to a stem which fits into the femur and replaces the head of the femur. A cement is used to secure the prosthetic devices to the bone.
Currently, the cement used for such replacement is polymethylmethacrylate (PMMA), which requires the use of a catalyst for hardening of the cement. The catalyst, which is supplied in liquid form, must be mixed thoroughly with the methyl methacrylate monomer, and other dry components of the cement, to ensure a complete and even hardening of the cement. Preferably the cement is mixed under a vacuum to prevent air bubbles from being trapped in the cement. Air bubbles in the final cement mix may result in a cement with an undesirably low tensile strength. Typically, mixing of the cement components for between 0.5 to 2 minutes is required to obtain a cement mixture of the desired consistency.
Currently, devices available for mixing of bone cement require hand mixing. This procedure is very laborious and requires a sustained physical effort to complete the mixing process. If the mixing process is not adequate or if the cement hardens before it is delivered, the cement must be discarded and an additional batch prepared. If the process has to be repeated, the surgical procedure, which is under way by the time the cement mixing is commenced, is delayed.
The bone cement is, and should remain sterile, therefore, a "sterile" person is required to perform the mixing and handling of all the sterile components of the cement preparation. With bone cement mixers currently available, a "non-sterile" person is also required, in addition to the "sterile" person, to assist in the set-up of the cement mixing apparatus and for handling of all nonsterile components involved in the mixing procedure. At the end of the mixing procedure an additional "sterile" person is often required to assist in the transfer of the cement into cartridges for delivery of the bone cement to the patient. Therefore, preparation of the cement requires at least two and often as many as three people.
The components of the pre-cured cement, methyl methacrylate monomer and catalyst, are irritants and potentially toxic. Therefore, inhalation of the cement components, particularly the monomer, is preferably avoided. Bone cement mixers currently available are not sealed throughout the mixing process and require that the components be manually dispensed into the bone cement mixer, thus exposing the operator to their fumes. These fumes may then be inhaled, for an extended period of time, by all personnel in the vicinity of the cement preparation, making cement preparation irritating and potentially hazardous to their health.
After the cement is mixed, it is poured into a cartridge for dispensing to the site for cementing. At this time, all instruments which have come in contact with the cement must be thoroughly cleaned or discarded. An additional problem with transferring the cement is the possibility of folding in air pockets that could negatively affect the cement/bone interface and reduce the cement's bonding strength.
It is desirable that an apparatus be provided which requires only a single operator to complete all the steps necessary to set-up the apparatus and to mix the cement. It is also desirable that the apparatus isolates all the fumes associated with the cement preparation to thus reduce exposure of hospital workers to these toxic compounds. It is also desirable that such an apparatus be motor driven to avoid the physical endurance required to mix cement and also to ensure reproducible mixing which reduces the need to dispose of cement batches that are unacceptable. It is also desirable that all parts of the device which come in contact with the cement are disposable so that clean-up is minimized.